We performed patch clamp experiments on bovine parathyroid cells in an attempt to explain the following unusual properties of these cells: (1) In contrast to other secretory cells, where the rate of secretion increases with increasing calcium concentration, the rate of hormonal secretion in parathyroid cells decreases when the external calcium concentration increases. (2) In contrast to other cells, parathyroid cells depolarize when the external calcium concentration increases. We found an unusual type of calcium-activated potassium channel in the plasma membrane of parathyroid cells - channels that tend to close when the internal calcium concentration is increased above about 150 nM, but which otherwise behave like the usual calcium-activated potassium channels. The unusual calcium-activated potassium channels we found can account for (2) above, since an increase in external calcium concentration causes an increase in internal calcium concentration, and this leads to closing of these unusual channels and consequent depolarization. These same channels together with the model for secretion that we presented last year might also account for (1) above, provided the channels were present in vesicle membranes as well as the plasma membrane of parathyroid cells. In order to test this point further, we are setting up a permeabilized cell system to determine the dependence of parathyroid secretion on internal calcium concentration over a wide range of concentrations.